What causes turbidity?

What causes turbidity?

Which is the maximum allowed turbidity in drinking water?

What are the consequences of high turbidity?

What are the impacts of turbidity?

How do we measure turbidity?

What is turbidity?

Turbidity is a measure of the degree to which the water loses its transparency due to the presence of suspended particulates.

The more total suspended solids in the water, the murkier it seems and the higher the turbidity.

Turbidity is considered as a good measure of the quality of water.

What causes turbidity?

There are various parameters influencing the cloudiness of the water. Some of these are:

- Phytoplankton

- Sediments from erosion

- Resuspended sediments from the bottom (frequently stir up by bottom feeders like carp)

- Waste discharge

- Algae growth

- Urban runoff

Which is the maximum allowed turbidity in drinking water?

The WHO (World Health Organization), establishes that the turbidity of drinking water shouldn't be more than 5 NTU, and should ideally be below 1 NTU.

What are the consequences of high turbidity?

The suspended particles absorb heat from the sunlight, making turbid waters become warmer, and so reducing the concentration of oxygen in the water (oxygen dissolves better in colder water). Some organisms also can’t survive in warmer water.

The suspended particles scatter the light, thus decreasing the photosynthetic activity of plants and algae, which contributes to lowering the oxygen concentration even more.

As a consequence of the particles settling to the bottom, shallow lakes fill in faster, fish eggs and insect larvae are covered and suffocated, gill structures get clogged or damaged…

What are the impacts of turbidity?

The main impact is merely esthetic: nobody likes the look of dirty water.

But also, it is essential to eliminate the turbidity of water in order to effectively disinfect it for drinking purposes. This adds some extra cost to the treatment of surface water supplies.

The suspended particles also help the attachment of heavy metals and many other toxic organic compounds and pesticides.

How do we measure turbidity?

Turbidity is measured in NTU: Nephelometric Turbidity Units. The instrument used for measuring it is called nephelometer or turbidimeter, which measures the intensity of light scattered at 90 degrees as a beam of light passes through a water sample.

The unit used in the ancient times was JTU (Jackson Turbidity Units), measured with the Jackson candle turbidimeter. This unit is no longer in standard use.

In lakes the turbidity is measured with a secchi disk (in the picture).

This is a black and white disk that is dropped in the water attached to a rope.

The depth that the disk reaches before it disappears from sight is recorded.

This provides an estimation of the turbidity level in the lake.